This invention relates to a battery having excellent strength and high heat dissipating characteristics.
As an example of the prior-art battery, a secondary battery employed for heavy loads is explained by referring to FIGS. 1 and 2.
In FIGS. 1 and 2, a conventional columnar-shaped battery shown in a perspective view and a cross-sectional view taken along line A--A in FIG. 1, respectively.
A columnar-shaped battery 30, a secondary battery used for heavy loads, usually has a columnar-shaped casing 31 as shown in FIG. 1. The inner structure of the battery 30 is shown in FIG. 2, in which a spiral-shaped electrode 34, formed by winding strip-shaped positive and negative electrodes with an interposed separator a large number of times around a center 35 is fitted in a cup-shaped negative electrode casing 31A, with a negative electrode lead 36 being welded to the negative electrode casing 31A and a positive electrode lead 37A being welded to a positive electrode lead 37B which is connected planar positive electrode plate 31B. After a liquid electrolyte is charged into the negative electrode casing 31A, the negative electrode casing 31A is caulked to the positive electrode casing 31B via a gasket 38 for sealing.
The columnar-shaped battery 30 having the above-described structure is subject to storage of heat and rise in temperature due to the heat of reaction of the electrodes resulting from charging/discharging and the Joule heat by the current flowing through the electrodes or the current collectors. Referring to FIG. 8, showing the temperature distribution for the columnar-shaped battery 30, the rise in temperature, which is decreased towards the outer peripheral surface of the battery 30, becomes maximum at its mid region 33, thus leading to shortened service life and occasionally to internal shorting or seal breaking.
Such phenomenon becomes outstanding when the battery is increased in size, For combatting such phenomenon, the cooling fluid may be caused to flow through the outside of the battery, or a liquid electrolyte within the battery may be connected to a heat sink and circulated by a pump. However, the energy density of the electric source system in its entirety is lowered due to the increase in volume or weight caused by the provision of the cooling system.
As means for solving the temperature related problem, it has been proposed to provide a structure in which heat exchange with air may be achieved easily on the battery surface or on the lead plate, such as by increasing the surface area of the battery.
The basic structure for increasing the surface area of the battery includes a planar structure or an elongated structure. However, if these structures are employed, the battery itself tends to be lowered in strength or to become deformed or bloated under external or internal pressure. Although the battery casing may be increased in thickness for increasing its strength, the energy density per unit area of the battery is thereby lowered.